Lucretius wrote about rheology in his De Rerum Natura (On the Nature of Things), Book II, ll. 391-397.[2] The above excerpt reads in Englishtranslation as,"We see how quickly through a colander the wines will flow; how, on the other hand, the sluggish olive oil delays: no doubt, because 'tis wrought of elements more large, or else more crook'd and intertangled. Thus it comes that the primordials cannot be so suddenly sundered one from other, and seep, one through each several hole of anything."[3] (Created using Inkscape. Click for larger image.)

"The resistance which arises from the lack of slipperiness of the parts of the liquid, other things being equal, is proportional to the velocity with which the parts of the liquid are separated from one another."

There's a simple scientific explanation for the Rayleigh–Taylor instability. In nature, a system tends to arrange itself to minimize energy. When lobes of one fluid interpenetrate another and a volume of a heavier fluid is pushed downwards and an equal volume of a lighter fluid is pushed upwards, the potential energy of the system is reduced since these fluid masses are affected by gravity. As Rayleigh found, the energy is further reduced as this interface disturbance grows under the influence of gravity, and Taylor generalized the action to accelerations other than just gravitational acceleration.

The Lehigh team perfected their measurement apparatus, a vane-spindle rheometer, over the course of several years.[6-7] Their research involved the incompressible Rayleigh-Taylor instability in elastic-plastic materials. Says Banerjee,

"There has been an ongoing debate in the scientific community about whether instability growth is a function of the initial conditions or a more local catastrophic process... Our experiments confirm the former conclusion: that interface growth is strongly dependent on the choice of initial conditions, such as amplitude and wavelength."[7]

In the Lehigh experiments, mayonnaise is used as an elastic-plastic material whose properties are much like that of the molten metal.[6-7] Wave-like perturbations were formed on the mayonnaise before acceleration, and a high-speed camera (500 frames/second) imaged the growth of the perturbations.[6-7]